1. Field of the Invention
This invention pertains to multioscillator ring laser gyros and particularly to an optical wedge used in a Faraday Cell therein.
2. Description of Related Art
The optical wedge is used in Faraday Cells in multioscillators. Preferably the path of the multioscillator is a rectangle, usually substantially square, that is folded about a diagonal into two planes. It should be noted, however, that some multioscillators have other than four path legs and mirrors and may be planar, and the invention using the corrected optical wedge described and claimed herein contemplates its use in such other ring lasers.
In this application, a "mode" is defined as a primary beam of a multioscillator ring laser gyro, plus two reflected double bounce beams produced inside an optical wedge by such primary beam. Thus, each multioscillator ring laser has four modes. Energizing the ring laser creates four primary beams, and they are resonant within the laser cavity. The two double-bounce beams of each of the modes are reflected and scattered beams within an optical wedge of a Faraday Cell, and they have very little energy compared to the energy in the primary beam because most of the beam is transmitted at the wedge-to-gas interface. The double-bounce beams create significant errors when the multioscillator is used to sense angular position, despite their low energy content.
The four primary beams of a multioscillator ring laser gyro may be slightly elliptically polarized, but with that understanding of their exact nature, for convenience let them be be described as circularly polarized. Polarize two of the primary beams with one polarity direction, and the other two beams with the other polarity direction. At each corner mirror of the ring laser, the polarity direction of each primary beam reverses, but since all beams reverse polarity it produces no confusion to ignore the polarity reversal.
Two of the primary beams traverse the ring laser in one direction, and the other two primary beams traverse the ring laser in the other direction. Arbitrarily designate the directions clockwise and counterclockwise. The primary beam combinations are arranged such that for each polarization polarity there is both a clockwise and an anticlockwise beam.
As a result of a nonplanar geometry and/or optical components such as a Faraday wedge, the beams are further displaced in frequency, one from another. The ring laser gain medium is capable of sustaining oscillations over a band of frequencies, and the four frequencies are within that band. The oscillations are produced by various means (not shown). For example, the oscillations may be produced by an electric d.c. gradient between a cathode and anode through part of the laser path; or the oscillations may be produced by an electromagnetic radio frequency field acting on a gas such as a neon gas mixture within a portion of the ring laser path. Designate the excitation region of the path the "gain bore".
In a multioscillator there operate two gyros. Two of the oppositely propagating beams produce signals for one gyro, and the other two oppositely propagating beams produce signals for the other gyro.
Because the two frequencies of each gyro do not coincide in the presence of zero angular velocity about the sensing axis of the gyro, each gyro is frequency biased. As the measured angular velocity increases, the two frequencies of one of the gyros diverge, and the two frequencies of the other of the gyros converge. The differences in sense of polarization allow the two gyro signals to be identified and converted into electrical signals which are measures of sensed angular velocity.
The output signal of a ring laser gyro is a heterodyne signal between the optical frequencies of the two beams comprising that gyro. The bias frequency is subtracted out, and the difference is proportional to the measured angular velocity. The use of a multioscillator avoids certain problems such as frequency locking of the two frequencies of the primary beams of a gyro when their difference frequency is small.
Designate the four beams herein, from lowest frequency to highest frequency, left circularly polarized anticlockwise, "La," left circularly polarized clockwise, "Lc," right circularly polarized clockwise, "Rc," and right circularly polarized anticlockwise, "Ra" beams.
There are means for extracting the beams and beating them against each other to produce signals which are a measure of angular velocity. For example, a portion of the two beams may be extracted and the extracted beam or beams folded and superimposed to produce fringes which are readily countable as they move across an optical field. It is most convenient to use the Lc, Ra beams for one gyro and the La, Rc beams for the other gyro.
There are three types of differential losses among the four lasing modes of a multioscillator ring laser gyro. When the left-circularly polarized modes (La, Lc) suffer different round-trip cavity loss from the right-circularly polarized modes (Ra, Rc) the differential loss is called, "differential polarization loss" (DPL).
When the clockwise modes (Lc, Rc) suffer different round-trip cavity loss from the anticlockwise modes (La, Ra) the differential loss is called, "differential directional loss" (DDL).
When the modes of one helicity (La,Rc) suffer different round-trip cavity loss than the modes of the other helicity (Lc,Ra) the differential loss is called, "magnetic circular dichroic loss" (MCDL).
Of the three types of loss, the multioscillator frequency bias is most sensitive, perhaps by a factor of a thousand, to MCDL. The operation of the invention could be used to reduce DDL and DPL, but for the best performance improvement in the multioscillator gyro, it is used to reduce MCDL.
Change in MCDL with temperature variation causes unwanted change in the multioscillator ring laser gyro frequency biases. Some of the frequency bias changes are such that their amplitudes vary monotonically with temperature, but others cycle, varying first upward or downward with changed temperature, then changing direction as the temperature further changes.
One possible way of removing all the differential loss effects (DPL, DDL and MCDL) caused by the double-bounce beams in the wedge would be to arrange the first and second double-bounce beams always to be anti-phased so their phasor sum is zero. In practice this is not likely because the scatter and reflection amplitudes from the two Faraday wedge faces are not equal, and because selection of the differential phase shifts between the two wedge faces, for scattering and reflection, likely cannot be guaranteed. Faraday-wedge fabrication techniques are believed not available to achieve the required accuracy.